Impulsive Brains

The brains of impulsive people are different from those who can delay gratification. It's all in the prefrontal cortex.

Children who easily yield to temptation often continue to do so after they grow up. That's the conclusion of a follow-up to a study begun in the late 1960s.

In the original study, children were a given a choice: have a cookie or marshmallow now, or hold off and get even more treats later. Forty years later, those who wanted instant gratification as a child continued to show less self-control as adults.

The most errors in the trial occurred when people were expecting a happy face and saw a frightened one.

Because sweet treats rarely have as much appeal to adults as they do to children, the follow-up used a different measure of self-control: the ability to refrain from pressing a button in response to an image on a computer monitor, as part of a go/no-go test.

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The study looked at 59 of the adults from the original study. The group was a mix of those who had tested high and those who had tested low on impulsivity. They were shown two different sets of 160 faces on a computer screen.

In the first (cool) set the images were male and female faces with neutral expressions. Participants were told to press a button when a face of one particular sex appeared, with 120 of the images designed to elicit a press (go) and 40 designed not to (no go). Since most faces led to a button press, the adults had to exercise some self-control to refrain from always pressing the button.

There was essentially no response difference between those who tested as highly impulsive as a child and those who did not. This is presumably because the test had no emotional content--it was a cool test.

Differences showed up in the second test, a hot version of the first, where the faces had happy or frightened expressions. Here, the same adults who wanted their treat immediately at the age of four made the most errors, inappropriately pressing the button. The most errors occurred in response to a frightened face when adults were expecting a happy face.

The researchers explain that in this test, the happy face took the place of the cookie as the adult's reward. Low delayers were unable to suppress their impulse to press the button. Psychological researchers are comfortable that go/nogo tests accurately show people's ability to control impulses. And there's no question that the most errors in the trial occurred when people were expecting a happy face and saw a frightened one.

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The researchers also showed that adults with the most self-control were using different parts of their brain during the test than those with less self-control. Repeating the test while the adults' brains were scanned by functional magnetic resonance imaging (fMRI) showed that the brain's prefrontal cortex was more active in high delayers, while the ventral striatum was more active in low delayers. This suggests a strong role for the prefrontal cortex in impulse control.

The researchers don't claim that impulsive children are fated to become impulsive adults. Everyone can be taught to control their impulses to some degree. What the study suggests is that that's how they tend to grow up without getting a little help.

An article on the study was published online ahead of print by Proceedings of the National Academy of Sciences (PNAS).